Method for protecting aircraft occupant and breathing mask

ABSTRACT

Method for protecting aircraft occupant comprising the steps of: providing a user ( 7 ) with a breathing mask ( 4 ) for aircraft occupant,—providing a respiratory gas ( 62 ) including a mixture of breathable gas and dilution gas to the user ( 7 ),—sensing partial pressure or rate of oxygen or carbon dioxide in exhalation gas ( 64 ) generated by the user ( 7 ),—adjusting ( 60 ) the rate of oxygen in the respiratory gas ( 62 ).

FIELD OF THE INVENTION

The present invention relates to a breathing mask for aircraft demandregulator and a dilution regulation method for protecting the occupant(passengers and/or crewmembers) of an aircraft against the risksassociated with high altitude depressurization and/or smoke and fume inthe cabin.

In particular, the invention relates to the adjustment of therespiratory gas supplied to a user to satisfy the needs of the user,using a source of breathable gas supplying pure oxygen (oxygen cylinder,chemical generator or liquid oxygen converter) or gas highly enriched inoxygen such as an on-board oxygen generator system (OBOGS).

To ensure the protection of the passengers and/or crewmembers in case ofdepressurization and/or occurrence of smoke in the aircraft, the demandregulators shall deliver a respiratory gas which is a mixture ofdilution gas (generally ambient air) and breathable gas depending ofcabin altitude. After a depressurization, the cabin altitude reaches avalue close to the aircraft altitude. The pressure value of the cabin isoften referred to as the cabin altitude. Cabin altitude is defined asthe altitude corresponding to the pressurized atmosphere maintainedwithin the cabin. This value differs from the aircraft altitude which isits actual physical altitude. Correspondence between pressure andconventional altitude are defined in tables. The minimum rate of oxygenin the respiratory gas according to the cabin altitude is set for civilaviation by the Federal Aviation Regulations (FAR).

BACKGROUND OF THE INVENTION

Most of the current crew breathing masks protecting aircraft crewmemberform hypoxia are equipped with oxygen regulators using pneumatictechnology for controlling using an open loop the partial pressure ofoxygen in the breathing gas. In this technology, ambient air is suckedthrough a dilution gas supply line by a Venturi which provides suctionby high velocity flow of breathable gas. An aneroid capsule (called alsoaltimeter capsule) regulates the altimetric oxygen enrichment byadjusting the section of the dilution gas supply line. Such demandregulators are known from the documents U.S. Pat. No. 6,994,086, FR 1484 691 or U.S. Pat. No. 6,796,306. As the oxygen enrichment depends onthe section of the dilution gas supply line controlled by the aneroidcapsule clearance, the oxygen consumption cannot be optimal for all ofthe cabin altitude range and/or for all of the breathing ventilation.

The need to save oxygen has lead to the development of electro-pneumaticregulator as described in the documents U.S. Pat. No. 4,336,590, U.S.Pat. No. 6,789,539, US 2007/0107729 or US 2009/0277449. These equipmentsperformed a close loop control of the breathing gas using a measure ofthe “inspired gas content”. These equipments which are interested onlyby the content of the gas provided to the pilot and not to thephysiological state of the pilot need fast sensor and actuator in orderto perform an accurate “real time” control of the inspired gas.

Other publication such as patent WO 2008/068545 uses the measure of thearterial blood oxygen saturation (SaO₂) in order to adjust the breathinggas content. This physiological parameter corresponds to the ratio ofthe amount of oxygen transported by the blood to the maximal theoreticalamount of gas transportable. It is linked to the oxygen partial pressurein the arterial blood (PaO₂) thanks to the Barcroft Curve or haemoglobindissociation curve shown in FIG. 1, which may vary depending on severalfactors such as the blood pH (saturation decreasing with pH), thepartial pressure of carbon dioxide in the alveoli PaCO2 (SaO₂ decreaseswhen PaCO₂ increases) and the temperature (SaO₂ decreases when the bloodtemperature increases).

PaO₂ is a difficult datum to measure on the opposite SaO₂ may be easilymeasure using a pulse oximeter. But once the PaO₂ reaches 80 hPa thecurve is almost flat, indicating there is little change in saturationabove this point. This is not a problem for passenger hypoxic protectionwhere the targeted PaO₂ level is below 80 hPa but this is not adaptedfor accurate crewmember hypoxic protection where the targeted PaO₂ levelis around 100 hPa.

SUMMARY OF THE INVENTION

The purpose of this invention is to provide a demand regulator which isreliable, quite cheap, simple to settle and supplies an oxygen rate incompliance with the minimum required while being close to the minimumrequired.

For this purpose the invention provides a method for protecting aircraftoccupant comprising the steps of:

-   -   providing a user with a breathing mask for aircraft occupant,    -   providing a respiratory gas including a mixture of breathable        gas and dilution gas to the user,    -   sensing partial pressure or rate of oxygen or carbon dioxide in        exhalation gas generated by the user,    -   adjusting the rate (fraction/percentage/concentration) of oxygen        (or breathable gas) in the respiratory gas.

The measurement of the oxygen partial pressure in the exhalation gasgives a quite good reliable estimation of the oxygen partial pressure inthe alveoli P_(A)O₂. This physiological parameter which expresses theoxygen partial pressure in the lung is close to the partial pressure inthe arterial blood P_(a)O₂ when the cabin altitude is high.

Use of the P_(A)O₂ for adjusting the rate of oxygen in the in therespiratory gas by controlling the dilution valve take into account thephysiology of the user which may differ between users. This allows amore accurate delivering of oxygen according to physiological need andregulation constraints. So, the risk of hypoxia of the aircraft occupant(in particular pilot or crewmember) and the consumption of oxygen can bereduced.

It should be noticed that rate, fraction, percentage or concentrationare different words referring to quite the same feature.

So, according to supplementary feature, the method preferably comprisesadjusting (regulating in closed loop) the rate of oxygen in therespiratory gas in accordance with the partial pressure or rate ofoxygen or carbon dioxide in the exhalation gas.

Therefore, the consumption of oxygen is optimised in function of therequirement of the user.

According to another feature, the method preferably comprises:

-   -   sensing partial pressure or rate of oxygen in exhalation gas        generated by the user, and    -   adjusting the rate of oxygen in the respiratory flow in        accordance with the partial pressure or rate of oxygen in        exhalation gas.

Indeed, it has appeared that adjusting the rate of oxygen in therespiratory flow in accordance with the partial pressure or rate ofoxygen in exhalation gas is more satisfying than in accordance with thepartial pressure or rate of carbon dioxide in exhalation gas.

However, according to a supplementary advantageous feature, the methodfurther comprises:

-   -   sensing partial pressure or rate of oxygen and carbon dioxide in        exhalation gas generated by the user, and    -   adjusting the rate of oxygen in the respiratory flow in        accordance with the partial pressure or rate of oxygen and        carbon dioxide in exhalation gas.

Indeed, partial pressure or rate of oxygen and carbon dioxide inexhalation gas generated by the user enables to further optimise theconsumption in oxygen, in particular by increasing the rate of oxygen inthe respiratory gas when the carbon dioxide partial pressure PCO₂ in theexhalation gas decreases under a determined threshold.

According to another feature the method preferably comprises:

-   -   sensing partial pressure or rate of oxygen in the exhalation gas        generated by the user,    -   sensing partial pressure or rate of oxygen in respiratory gas,        and    -   determining coherence between the partial pressure or rate of        oxygen sensed in the exhalation gas and the partial pressure or        rate of oxygen in respiratory gas sensed for detecting failure        (in particular a failure in the dilution adjusting device).

This check is much more accurate and more reliable than usual checkconsisting in out of range alarm on the oxygen sensor for monitoringfailure in the regulating process

According to supplementary feature in accordance with the invention,preferably the method further has the following steps:

-   -   sensing barometric pressure in the aircraft, and    -   determining coherence between the partial pressure or rate of        oxygen sensed in the exhalation gas and the partial pressure or        rate of oxygen in respiratory gas thanks to a coherence equation        including:        -   the partial pressure or rate of oxygen sensed in the            exhalation gas,        -   the partial pressure or rate of oxygen in respiratory gas,            and        -   the barometric pressure.

The relation between these features enables to determine a failure quiteeasily and is particularly efficient.

According to another supplementary feature in accordance with theinvention, preferably said coherence equation is:

P_(A)O₂=F₁O₂.(P_(B)−P_(A)H₂O)−P_(A)CO₂.(F₁O₂+(1−F₁O₂)/R),

with:

P_(A)O₂ is the oxygen partial pressure sensed in the exhalation gas,

P_(B) is the barometric pressure in the aircraft,

P_(A)CO₂ is the partial pressure of carbon dioxide in the exhalationgas,

P_(A)H₂O is the partial pressure of water in the exhalation gas,

F₁O₂ is rate of oxygen or the partial pressure of oxygen sensed in therespiratory gas,

R is a constant between 0.1 and 1.2 corresponding to respiratoryquotient.

According to another supplementary feature in accordance with theinvention, preferably the method further comprises sensing the partialpressure of carbon dioxide in the exhalation gas.

The determination of failure is more accurate.

According to another supplementary feature in accordance with theinvention, preferably the partial pressure of water in the exhalationgas is replaced by a constant.

According to another feature in accordance with the invention, themethod comprises (alternatively) sensing the partial pressure or rate ofoxygen in the exhalation gas and the partial pressure or rate of oxygenin respiratory gas sensed with a sole (the same) gas sensor.

The determination of failure is reliable while requiring few elements(means).

The invention also relates to a breathing mask for aircraft occupantincluding a demand regulator, said regulator comprising:

-   -   a breathable gas supply line to be connected to a source of        breathable gas and supplying a flow chamber with breathable gas,    -   a dilution gas supply line to be connected to a source of        dilution gas and supplying the flow chamber with dilution gas,    -   a dilution adjusting device adjusting the rate of dilution gas        in the respiratory gas supplied to the flow chamber, the        dilution adjusting device comprising a dilution valve, a gas        sensor adapted to sense partial pressure or rate of oxygen or        carbon dioxide and a control device controlling the dilution        valve in accordance with a dilution signal generated by the gas        sensor in function of the partial pressure or rate of oxygen or        carbon dioxide.

In advantageous embodiments, the breathing assembly preferably furtherhas one or more of the following features:

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will appear inthe following detailed description, with reference to the appendeddrawings in which:

FIG. 1 represents the arterial blood saturation in accordance with thepartial pressure of oxygen in the arterial blood,

FIG. 2 shows a breathing mask comprising a flow chamber,

FIG. 3 schematically represents a first flow and a second flow in theflow chamber of the breathing mask, according to first embodiment of asensing device,

FIG. 4 represents variations of the first flow in the flow chamberduring the time,

FIG. 5 represents variations of the second flow in the flow chamberduring the time,

FIG. 6 represents measurements provided by gas sensors placed in theflow chamber,

FIG. 7 represents a second embodiment of a sensing device in accordancewith the invention,

FIG. 8 represents a third embodiment of a sensing device in accordancewith the invention,

FIG. 9 represents a fourth embodiment of a sensing device in accordancewith the invention,

FIG. 10 represents a fifth embodiment of a sensing device in accordancewith the invention,

FIG. 11 represents a step of a method according to the invention usingthe sensing device of the fifth embodiment,

FIG. 12 is a flowchart representing different steps of a method forusing the sensing device of the fifth embodiment,

FIG. 13 represents partial pressure of oxygen according to the methodfor using the sensing device of the fifth embodiment,

FIG. 14 represents partial pressure of oxygen according to analternative method for using the sensing device of the fifth embodiment.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 discloses main functions of a breathing mask 4 for occupant of anaircraft, in particular for pilot disposed in a cabin 10 of an aircraft.

The breathing mask 4 comprises a demand regulator 1 and an oronasal facepiece 3 fixed to a tubular connecting portion 5 of the regulator 1. Whena user 7 dons the breathing mask 4, the oronasal face piece 3 is put tothe skin of the user face 7 and delimits a respiratory chamber 9.

The demand regulator 1 has a casing 2 including a breathable gas supplyline 12, a dilution gas supply line 14 and a respiratory gas supply line16. The respiratory gas supply line 16 has a downstream end in fluidcommunication with the respiratory chamber 9.

The breathable gas supply line 12 is supplied at its upstream end withpressurized oxygen by a source of breathable gas 8 through a feedingduct 6. In the embodiment shown, the pressurized source of breathablegas 8 is a cylinder containing pressurized oxygen. The breathable gassupply line 12 supplies the respiratory chamber 9 with breathable gasthrough the respiratory gas supply line 16, the downstream end of thebreathable gas supply line 12 being directly in fluid communication withthe upstream end of the respiratory gas supply line 16.

The dilution gas supply line 14 is in communication by its upstream endwith a source of dilution gas. In the illustrated embodiment, thedilution gas is air and the source of dilution gas is the cabin 10 ofthe aircraft. The dilution gas supply line 14 supplies the respiratorychamber 9 with dilution gas through the respiratory gas supply line 16,the downstream end of the dilution gas supply line 14 being directly influid communication with the upstream end of the respiratory gas supplyline 16. So, in the embodiment illustrated in FIG. 2, the breathable gasand the dilution gas are mixed in the respiratory gas supply line 16 ofthe casing 2, i.e. before supplying the respiratory chamber 9 throughthe tubular connecting portion 5. Therefore a flow 62 of respiratory gasflows in the respiratory gas supply line 16 and the respiratory chamber9, the respiratory gas including breathable gas and dilution gas mixed.

The regulator 1 further comprises an exhaust line 18 and an exhaustvalve 20. The exhaust valve 20 is disposed between the downstream end ofthe exhaust line 18 and the cabin 10 (ambient air). The upstream end ofthe exhaust line 18 is in communication with the respiratory chamber 9of the oronasal face piece 3 through the tubular connecting portion 5and receives a flow 64 of gas exhaled by the user. Concerning theexhaust of the exhalation gas 64, the exhaust valve 20 functions as acheck valve which opens under the pressure of the exhalation gas 64 andcloses for preventing air of the cabin 10 from entering into the flowchamber 30.

The user 7 breathes in and breathes out in the respiratory chamber 9.The exhalation line 18 is in communication directly or through therespiratory chamber 9 with the respiratory gas supply line 16.Therefore, the gas supply line 16, the respiratory chamber 9 and theexhalation line 18 define a flow chamber 30 without separation.

The demand regulator 1 further has a pressure adjusting device 22 and adilution adjusting device 24.

The pressure adjusting device 22 adjusts the pressure in the flowchamber 30 and in particular in the respiratory chamber 9. In theembodiment illustrated in FIG. 2, the pressure adjusting device 22comprises in particular a main valve disposed between the feeding duct 6and the respiratory gas supply line 16.

The dilution adjusting device 24 adjusts the rate of oxygen in therespiratory gas flow 62. In the embodiment illustrated, the dilutionadjusting device comprises in particular a dilution valve 23, a controldevice 60, a flow direction sensor 38, an oxygen sensor 42, an optionalcarbon dioxide sensor 68, a cabin altitude sensor 71 and an optionalaircraft altitude sensor 72. The dilution valve 23 is disposed betweenthe dilution gas supply line 14 and the respiratory gas supply line 16.The control device 60 controls the dilution valve 23. The flow directionsensor 38, the oxygen sensor 42, the carbon dioxide sensor 68, the cabinaltitude sensor 71 and the aircraft altitude sensor 72 provideinformation to the control device 60 to adjust the rate of oxygen in therespiratory gas 62 by actuating the dilution valve 23. The cabinaltitude sensor 71 senses the barometric pressure, i.e. the ambient(absolute) pressure (in the cabin 10 of the aircraft). The aircraftaltitude sensor 72 senses the pressure outside the cabin 10. Duringnormal operation, equipment pressurises the cabin 10 at the cabinaltitude, so the pressure is higher than the pressure outside the cabinand conversely the cabin altitude is lower than the aircraft altitude.

Demand regulators start supplying first gas mixture (respiratory gas) inresponse to the user of the breathing mask breathing in and stopssupplying respiratory gas when the user stops breathing in.

One can refers to prior art, such as for example to document U.S. Pat.No. 6,789,539 for a more detailed description of a demand regulator. Thepresent invention is also applicable to other types of dilutionadjusting device 24, such as the dilution adjusting device disclosed inpatent application PCT/IB2011/000772 or U.S. 6,789,539 included byreference.

FIG. 3 schematically represents a sensing device 100 comprising a flowdirection sensor 38, two gas sensors: an oxygen sensor 42 and anoptional carbon dioxide sensor 68. The sensing device 100 is a portionof the breathing mask 4 represented in FIG. 2. The oxygen sensor 42 andthe carbon dioxide sensor 68 are placed in the flow chamber 30 forming asensing chamber 40 in which alternatively flows a first gas mixture 32and a second gas mixture 34. In order to adjust the rate of oxygen todeliver to the user 7, a characteristic (in particular the partialpressure or percentage of a gaseous) of a gaseous constituent (inparticular oxygen or carbon dioxide) of at least the first gas mixture32 is to be detected by the oxygen sensor 42 and the carbon dioxidesensor 68.

The flow direction sensor 38, the oxygen sensor 42 and the carbondioxide sensor 68 are connected to the control device 60. The flowdirection sensor 38 detects if the flow direction in the flow chamber 30corresponds to the direction of the first flow mixture 32. The flowdirection sensor 38 may also detect if the flow direction in the flowchamber 30 corresponds to the direction of the second flow mixture 34.

Indeed, the first gas mixture 32 may be either the respiratory gas 62 orthe exhalation gas 64, which means that the characteristic of thegaseous constituent to sense may be either in the respiratory gas or inthe exhalation gas. So, the first gas mixture 32 flows from the tubularconnecting portion 5 to (the mouth or nose of) the user 7 or from theuser 7 to the tubular connecting portion 5. Conversely, the second gasmixture 34 may be either the exhalation gas 64 or the respiratory gas62.

The oxygen sensor 42 is adapted to determine in particular partialpressure (or percentage) in oxygen of the gas contained in the sensingchamber 40 whereas the carbon dioxide sensor 68 is adapted to determinein particular partial pressure (or percentage) in carbon dioxide of thegas contained in the sensing chamber 40.

The flow direction sensor 38 includes in particular a pressure sensor, apressure gauge sensor, a pressure differential sensor, thermistances, asensor of the state of a check valve or a piezo sensor device comprisinga flexible sheet and detecting the direction of the curvature of theflexible sheet.

As represented schematically in FIG. 4, between the time 0 and the timeT₁, the gas content in the flow chamber 30 reaches the gas content ofthe first gas mixture flow 32 and then between the time T₁ and the timeT₁₊T₂, the first gas mixture flow 32 becomes absent from the flowchamber 30.

As represented schematically in FIG. 5, between the time 0 and the timeT₁, the second gas mixture flow 34 becomes absent from the flow chamber30 and then, between the time T₁ and the time T₁₊T₂, the gas content inthe flow chamber 30 reaches the gas content of the second gas mixtureflow 34.

It should be noticed that in FIGS. 4 and 5 the time for filing the flowchamber 30 is neglected.

So, it may be considered by simplification that successively during a T₁period the first gas mixture 32 flows in the flow chamber 30 in a firstdirection, then during a T₂ period the second gas mixture 34 flows intothe flow chamber 30 in a second direction opposite to the firstdirection, then the first gas mixture 32 flows again in the flow chamber30 during another T₁ period, and so on. The T₁ period may be consideredas equal to the T₂ period, and called T.

The gaseous content of the first gas mixture 32 being different from thesecond gas mixture 34, the second gas mixture 34 disturbs themeasurement of the characteristic of the gaseous content of the firstgas mixture 32. It should be understood that the first gas mixture andthe second gas mixture may content the same constituents (at least someidentical constituents), and only differ in the percentage of some ofthe constituents (in particular percentage of oxygen, carbon dioxide andsteam).

FIG. 6 presents three measurements 42a, 42 b, 42 c provided by oxygensensors 42 having different response times Tr for the above describedexample. The measurements 42 a, 42 b, 42 c correspond to oxygen sensorshaving a response time respectively equal to T/10, T/2 and 2T.

It appears that the oxygen sensor providing measurements 42 a, 42 b aresuitable for the present example. Therefore, when the flow directionsensor 38 detects the exhalation gas 64, the oxygen sensor 42 determinesthe partial pressure (or percentage) in oxygen in the exhalation gas 64and conversely when the flow direction sensor 38 detects the respiratorygas 62, the oxygen sensor 42 determines the partial pressure (orpercentage) in oxygen in the respiratory gas 62. Therefore, the oxygensensor 42 provides the control device 60 with the oxygen partialpressure in the exhalation gas 64 and with the oxygen partial pressurein the respiratory gas 62. As the cabin altitude sensor 71 provides thecontrol device 60 with the barometric pressure (total pressure in thecabin 10), the control device 60 determines the fraction of oxygen inthe respiratory gas, since the oxygen partial pressure in therespiratory gas is equal to the product of the barometric pressure andthe fraction of oxygen in the respiratory gas.

The oxygen sensor providing measurement 42 c is not appropriate. So, theshorter the response time of the gas sensor is, the more accurate themeasurement is. But, a gas sensor with a short time response isgenerally more expensive than a sensor with a longer time response, andsometimes a gas sensor with a time response satisfying for a particularapplication does not exist.

FIG. 7 represents a second embodiment of a sensing device 100 inaccordance with the invention. The sensing device 100 comprises a flowdirection sensor 38, a shutter 50, a driving device 51 and an oxygensensor 42 placed in a sensing chamber 40 in fluid communication with theflow chamber 30 through a passage 66. A carbon dioxide sensor 68 may beplaced in the sensing chamber 40 instead of the oxygen sensor 42 or inaddition to the oxygen sensor 42, in order to determine in particularpartial pressure (or percentage) in carbon dioxide of the gas containedin the sensing chamber 40.

The flow direction sensor 38 and the oxygen sensor 42 are connected tothe control device 60. The flow direction sensor 38 detects if the flowdirection in the flow chamber 30 corresponds to the direction of thefirst flow mixture 32. In variant, the flow direction sensor 38 maydetect if the flow direction in the flow chamber 30 corresponds to thedirection of the second flow mixture 34.

The shutter 50 is movable between an active position in which it closesthe passage 66 and an inactive position in which it is away from thepassage 66.

The control device 60 controls the driving device 51 in order to placethe shutter 50 in open position when the flow direction sensor 38detects the first gas flow 32, so that the first gas mixture flow 32(partially) enters in the sensing chamber 40. Moreover, the controldevice 60 controls the driving device 51 in order to place the shutter50 in closed position when the flow direction sensor 38 does not detectthe first gas flow 32, so that the second the second gas mixture flow 34is prevented from entering in the sensing chamber 40.

Therefore, the sensing chamber 40 contains only gas mixture of the firstgas mixture flow 32 at any time. So, the oxygen sensor 42 transmits adilution signal which accuracy is not influenced by the second gasmixture flow 34. The control device 60 controls the dilution valve 24 inaccordance with the dilution signal generated by the oxygen sensor 42.

The oxygen sensor 42 is adapted to determine in particular partialpressure (or percentage) in oxygen of the gas contained in the sensingchamber 40.

The flow direction sensor 38 includes in particular a pressure sensor, apressure gauge sensor, a pressure differential sensor, thermistances, asensor of the state of a check valve or a piezo sensor device comprisinga flexible sheet and detecting the direction of the curvature of theflexible sheet.

FIG. 8 represents a third embodiment of a sensing device 100 inaccordance with the invention.

In this third embodiment, the characteristic of the gaseous constituentto sense is in the respiratory gas 62, so that the first gas mixtureflow 32 is the respiratory gas flow and the second gas mixture flow 34is the exhalation gas flow.

An isolation valve 36 is inserted between the respiratory gas supplyline 16 and the respiratory chamber 9. The oxygen sensor 42, inconnection with the control device 60, is placed in the respiratorychamber 16 which forms the sensing chamber 40. The isolation valve 36prevents gas from entering into the sensing chamber 16, 40 from therespiratory chamber 9. In an alternative embodiment, the flow directionsensor 38 may detect if the flow direction in the flow chamber 30corresponds to the direction of the second flow mixture 34.

In the embodiment illustrated, the isolation valve 36 is a check valve.In variant, it may be an inspiration valve similar to the exhaust valve20.

FIG. 9 represents a fourth embodiment of a sensing device 100 inaccordance with the invention.

In this fourth embodiment, the characteristic of the gaseous constituentto sense is in the exhalation gas, so that the first gas mixture flow 32is the exhalation gas flow 64 and the second gas mixture flow 34 is therespiratory gas flow 62.

An isolation valve 36 is inserted between the respiratory chamber 9 andthe exhalation line 18. The oxygen sensor 42, in connection with thecontrol device 60, is placed in the exhalation line 18 which forms thesensing chamber 40. The isolation valve 36 prevents gas from enteringinto the respiratory chamber 9 from the exhalation line 18. The carbondioxide sensor 68 may be placed in the sensing chamber 40 instead of theoxygen sensor 42 or in addition to the oxygen sensor 42.

In the embodiment illustrated, the isolation valve 36 is a check valve.In variant, it may be an inspiration valve similar to the exhaust valve20.

FIG. 10 represents a fifth embodiment of a sensing device 100 inaccordance with the invention.

The oxygen sensor 42 comprises a pumping plate 44, a first disk of solidionic conductor 45, a common plate 46, a second disk of solid ionicconductor 47 and a sensing plate 48.

The pumping plate 44, the common plate 46 and the sensing plate 48 areelectrodes preferably made of platinum films.

The pumping plate 44, the common plate 46 and the sensing plate 48 areof substantially annular form. Therefore, the sensing chamber 40 isdelimited by the common plate 46, the first ionic conductor 45 and thesecond ionic conductor 47.

A current source 39 is inserted between the pumping plate 44 and thecommon plate 46. The common plate 46 and the sensing plate 48 areconnected to the control device 60, as well as the flow direction sensor38.

The pumping plate 44, the first solid ionic conductor 45 and the commonplate 46 define a pumping electrochemical cell 56. The common plate 46,the second solid ionic conductor 47 and the sensing plate 48 define asensing electrochemical cell 58.

The ionic conductors 45, 47 define solid electrolyte. They arepreferably made in dioxide zirconium suitably adapted for the conductionof ions of oxygen O₂.

The oxygen sensor 42 further comprises an optional filter 49 surroundingthe pumping electrochemical cell 56 and the sensing electrochemical cell58. The filter 49 prevents particles from entering into the sensor 42.Therefore, the oxygen sensor 42 includes a buffer chamber 41 extendingbetween the flow chamber 30 and the pumping electrochemical cell 56 (andthe sensing electrochemical cell 58).

The oxygen sensor 42 may be placed either in the respiratory chamber 9,in the respiratory gas supply line 16 or in the exhalation line 18, andof any of the first to fourth embodiment described above.

As illustrated in FIG. 11, when the electrical power supply 39 outputs apumping current i at the value Ip, oxygen ions are transported throughthe ionic conductors 45 from the sensing chamber 40 to the bufferchamber 41. Therefore, an evacuation phase 28 corresponds to a phase ofpumping current i equal to Ip. So, the partial pressure in Oxygen PO₂ inthe sensing chamber 40 decreases. The voltage Vs between the sensingplate 48 and the common plate, called Nerst voltage, increases.

When the electrical power supply 39 outputs a pumping current i at thevalue −Ip, oxygen ions are transported through the ionic conductor 45from the buffer chamber 41 to the sensing chamber 40. Therefore, apressurisation phase 26 corresponds to a phase of pumping current iequal to −Ip. So, the partial pressure in Oxygen PO₂ in the sensingchamber 40 increases and the Nerst voltage Vs between the sensing plate48 and the common plate 46 decreases.

In operation, the control device 60 causes a repetitive sequence wherethe oxygen pumping current I is successively reversed to maintain theNerst voltage Vs between to predetermined values V₁, V₂.

Therefore, the partial pressure of Oxygen in the sensing chamber 40varies between two values PO₂low and PO₂high.

The period of oscillation Tp is proportional to the oxygen partialpressure in the buffer chamber 41. Therefore, period of the pumpingcycle is used to determine the ambient oxygen partial pressure.

The transportation of the oxygen through the ionic conductor 45 duringthe pressurisation phase 26 creates a pressure drop in the bufferchamber 41. The low porosity of the external filter 49 limits the entryof the ambient gas into the sensor and is responsible of the main delay(high response time) in the oxygen partial pressure measurement.

The response time of the oxygen sensor 42 generates an error in themeasurement of the oxygen partial pressure in the first gas mixture flow32, due to the second gas mixture flow 34.

As shown in FIG. 12, in order to limit the error in the measurement ofthe oxygen partial pressure in the first gas mixture flow 32, thedirection of the flow in the flow chamber 30 is sensed by the directiongas sensor 38. During step S38, based on the signal provided by the flowdirection sensor 38, the control device 60 determines if the flow in theflow chamber 30 is in the direction of the first gas mixture flow 32. IfYes, during a measurement period 52, the pressurization phase 26 and theevacuation phase 28 repetitively and alternatively follow one another,as shown in FIGS. 13 and 14. If No, as shown in FIG. 13, during a periodwithout measurement 54, the pressurisation of the sensing chamber 40 isstopped, no pressurisation phase 26 occurring during the period withoutmeasurement 54. Consequently, diffusion of the second gas mixture flow34 into the gas sensor buffer 41 is reduced and the sensing accuracy ofthe oxygen sensor 42 is improved. For example, the gas sensormeasurement process is active during inspiration of the user and stoppedduring exhalation of the user if the characteristic of the gaseouscomponent to be sensed is in the respiratory gas.

In a variant shown in FIG. 14, during the period without measurement 54,preferably at the beginning, an evacuation phase 28 is achieved. Duringthe evacuation phase 28 of the period without measurement 54, as shownin FIG. 14, the pumping current i is preferably lower than during theevacuation phase 28 of the measurement period 52, i.e. lower than Ip.Therefore, the evacuation phase 28 of the period without measurement 54lasts during all the period without measurement 54 or at least more thanhalf of the period without measurement 54.

Moreover, the respiratory gas 62 and the exhalation gas 64 arepreferably successively (alternatively) considered as the first gasmixture flow 32 and the gas second mixture flow so that the oxygenpartial pressure is successively measured in the respiratory gas 62 andthe exhalation gas 64.

Since the oxygen partial pressure in the respiratory gas is equal to theproduct of the barometric pressure sensed by the cabin altitude sensor71 and the fraction of oxygen in the respiratory gas 62, the controldevice 60 determines the fraction of oxygen in the respiratory gas 62and the oxygen partial pressure in the exhalation gas 64.

Concerning the operating of the regulator 1 using the, the dilutionadjusting device 24 adjusts the rate of oxygen in the respiratory gas 62in accordance with the oxygen partial pressure PO₂ or rate of oxygen inthe exhalation gas 64, sensed by the oxygen sensor 42 of one of thesensing devices 100 above described.

It should be noticed that the oxygen sensors currently available canprovide directly either the oxygen partial pressure or the rate ofoxygen, and that oxygen partial pressure PO₂ is equal to the rate ofoxygen multiplied by the barometric pressure sensed by the cabinaltitude sensor 71.

The dilution valve 23 is preferably controlled in closed loop with aProportional Integral Derivative (PID) controller included in thecontrol device 60, in order to adjust the oxygen partial pressure PO₂ inthe exhalation gas 64 sensed by the oxygen sensor 42 in accordance withthe cabin altitude sensed by the cabin altitude sensor 71, optionally inaccordance with the aircraft altitude sensed by the aircraft altitudesensor 72 and preferably in accordance with the carbon dioxide partialpressure PCO₂ in the exhalation gas 64 sensed by the carbon dioxidesensor 68. Preferably, the rate of oxygen in the respiratory gas 62 hasto be increased when the carbon dioxide partial pressure PCO₂ in theexhalation gas 64 decreases under a determined threshold.

The measurement of the oxygen partial pressure in the exhalation gas 64gives a quite good reliable estimation of the oxygen partial pressure inthe alveoli P_(A)O₂. This physiological parameter which expresses theoxygen partial pressure in the lung is close to the partial pressure inthe arterial blood P_(a)O₂ when the cabin altitude is high.

Use of the P_(A)O₂ for adjusting the rate of oxygen in the in therespiratory gas 62 by controlling the dilution valve take into accountthe physiology of the user which may differ between users. This allows amore accurate delivering of oxygen according to physiological need andregulation constraints. So, the risk of hypoxia of the aircraft occupant(in particular pilot or crewmember) and the consumption of oxygen can bereduced.

Moreover the content of respiratory gas delivered by the dilutionadjusting device 24, 38, 42, 60 is diluted inside the lung capacity. Asthe P_(A)O₂ is a “slow” variable needing several breathing cycles beforechange, the dynamic of the dilution adjusting device 24, 38, 42, 60using a close loop control may be very slow (around 0.1 Hz).Consequently this will simplify dilution valve 23 and the oxygen sensor42.

The adjusting device 24 and in particular dilution valve may beadvantageously replaced by at least one more sophisticated adjustingdevice such as disclosed in the patent application PCT/IB2011/000772incorporated herein by reference.

Otherwise, the control device determines coherence between the fractionof oxygen in the respiratory gas 62 and the oxygen partial pressure inthe exhalation gas 64. As mentioned above the control device 60determines the fraction of oxygen in the respiratory gas 62 and theoxygen partial pressure in the exhalation gas 64. Moreover, the fractionof oxygen in the respiratory gas 62 and the oxygen partial pressure inthe exhalation gas 64 are linked by the following alveolar gas equation:

${{P_{A}O_{2}} = {{F_{I}{O_{2}\left( {P_{B} - {P_{A}H_{2}O}} \right)}} - {P_{A}{{CO}_{2}\left( {{F_{I}O_{2}} + \frac{1 - {F_{I}O_{2}}}{R}} \right)}}}},$

with

P_(A)O₂ is the partial pressure of oxygen in the alveolar gas

P_(B) is the barometric pressure in the cabin 10 of the aircraft

P_(A)CO₂ is the partial pressure of carbon dioxide in the exhalation gas

P_(A)H₂O is the partial pressure of water in the exhalation gas

F₁O₂ is the rate of oxygen in the respiratory gas 62

R is a constant corresponding to respiratory quotient.

The partial pressure of oxygen in the alveolar gas may be approximatedto partial pressure of oxygen in the exhalation gas 64.

The partial pressure of carbon dioxide P_(A)CO₂ in the exhalation gas 64is preferably sensed by the carbon dioxide sensor 68. Otherwise, thepartial pressure of carbon dioxide P_(A)CO₂ may be replaced by aconstant close to 53 hPa, as it is generally quite close to this value.

The partial pressure of water P_(A)H₂O is in the exhalation gas 64 maybe replaced by a constant close to 63 hPa at the temperature of thealveolar gas (estimated to 37° C.).

R may be estimated between 0.1 and 1.2, preferably close to 0.83 innormal conditions.

So, the alveolar gas equation may be simplified into a followingcoherence equation:

P_(A)O₂=F₁O₂.(P_(B)−K₁)−P_(A)CO₂.(F₁O₂+(1−F₁O₂)/K₂),

with K₁, K₂ and K₃ constants or further simplified into:

P_(A)O₂=F₁O₂.(P_(B)−K₁)−K₃.(F₁O₂+.(1−F₁O₂)/K₂).

Failure is determined by comparison with a range value with a ratiobetween the measured value and the value estimated (partial pressure ofoxygen in the in the exhalation gas 64 or the rate of oxygen in therespiratory gas 62) by the coherence equation. In case of failuredetermined a warning alarm is activated.

A data consistency check in real time monitoring of the elements of thedilution adjusting device 24 is therefore performed. This check is moreaccurate and more reliable than usual check consisting in out of rangealarm on the oxygen sensor for monitoring failure in the regulatingprocess. Indeed, with usual check if the ratio between the real pressureand the pressure sensed may be high before being detected.

Preferably, the partial pressure of oxygen in the exhalation gas 64 issensed with the same gas (oxygen) sensor 42 as the oxygen sensor 42which enables the control device 60 to determine the rate of oxygen inthe respiratory gas 62 by sensing the partial pressure of oxygen in therespiratory gas 62.

Indeed, if a failure occurs concerning the oxygen sensor 42, since thereis a ratio substantially away from 1 between the oxygen partial pressurein the respiratory gas 62 and the oxygen partial pressure in theexhalation gas 64, and since the above coherence equation is not linear,the failure of the oxygen sensor 42 should be detected.

1. Method for protecting aircraft occupant comprising the steps of:providing a user with a breathing mask for aircraft occupant, providinga respiratory gas including a mixture of breathable gas and dilution gasto the user, sensing partial pressure or rate of oxygen in respiratorygas, adjusting the rate (fraction/percentage/concentration) of oxygen(or breathable gas) in the respiratory gas, further comprising: sensingpartial pressure or rate of oxygen in the exhalation gas generated bythe user, determining coherence between the partial pressure or rate ofoxygen sensed in the exhalation gas and the partial pressure or rate ofoxygen in respiratory gas sensed for detecting failure.
 2. The methodaccording to claim 1, comprising: adjusting the rate of oxygen in therespiratory gas in accordance with the partial pressure or rate ofoxygen or carbon dioxide in the exhalation gas.
 3. The method accordingto claim 1, comprising: sensing partial pressure or rate of oxygen inexhalation gas generated by the user, adjusting the rate of oxygen inthe respiratory flow in accordance with the partial pressure or rate ofoxygen in exhalation gas.
 4. The method according to claim 3,comprising: sensing partial pressure or rate of oxygen and carbondioxide in exhalation Gas generated by the user, adjusting the rate ofoxygen in the respiratory flow in accordance with the partial pressureor rate of oxygen and carbon dioxide in exhalation gas.
 5. (canceled) 6.The method according to claim 1 further comprising: sensing barometricpressure in the aircraft and determining coherence between the partialpressure or rate of oxygen sensed in the exhalation gas and the partialpressure or rate of oxygen in respiratory gas thanks to a coherenceequation including: the partial pressure or rate of oxygen sensed in theexhalation gas, the partial pressure or rate of oxygen in respiratorygas, and the barometric pressure.
 7. The method according to claim 6wherein said coherence equation is:P_(A)O₂=F₁O₂.(P_(B)−P_(A)H₂O)−P_(A)CO₂.(F₁O₂+(1−F₁O₂)/R), with P_(A)O₂is the oxygen partial pressure sensed in the exhalation gas P_(B) is thebarometric pressure in the aircraft P_(A)CO₂ is the partial pressure ofcarbon dioxide in the exhalation gas P_(A)H₂O is the partial pressure ofwater in the exhalation gas F₁O₂ is rate of oxygen or the partialpressure of oxygen sensed in the respiratory gas (62) R is a constantbetween 0.1 and 1.2 corresponding to respiratory quotient.
 8. The methodaccording to claim 7 further comprising: sensing the partial pressure ofcarbon dioxide in the exhalation gas.
 9. The method according to claim 7wherein the partial pressure of water in the exhalation gas is replacedby a constant.
 10. The method according to claim 1 wherein the coherenceis determined by comparison of a range value and a ratio between themeasured value and the value estimated by the coherence equation. 11.The method according to claim 1 further comprising sensing the partialpressure or rate of oxygen in the exhalation gas and the partialpressure or rate of oxygen in respiratory gas sensed with a sole (thesame) gas sensor.
 12. A breathing mask for aircraft occupant including ademand regulator, said regulator comprising: a breathable gas supplyline to be connected to a source of breathable gas and supplying a flowchamber with breathable gas, dilution gas supply line to be connected toa source of dilution gas and supplying the flow chamber with dilutiongas, a dilution adjusting device adjusting the rate of dilution gas inthe respiratory gas supplied to the flow chamber, the dilution adjustingdevice comprising a dilution valve, a gas sensor adapted to sensepartial pressure or rate of oxygen or carbon dioxide and a controldevice controlling the dilution valve in accordance with a dilutionsignal generated by the gas sensor in function of the partial pressureor rate of oxygen or carbon dioxide, wherein: the dilution adjustingdevice further comprises a gas flow direction sensor, the control devicecomprises a pump electrochemical cell placed in the flow chamber foralternatively sensed the oxygen partial pressure in the respiratory gasand in the exhalation gas.
 13. The breathing mask according to claim 12wherein the gas sensor is adapted to sense partial pressure or rate ofoxygen or carbon dioxide in exhalation gas generated by the user. 14.The breathing mask according to claim 12 wherein the gas sensor isadapted to sense partial pressure or rate of oxygen.
 15. The breathingmask according to claim 14 wherein the dilution adjusting device furthercomprises a carbon dioxide gas sensor.
 16. (canceled)
 17. (canceled)